Optical recording disks and other optical recording media have attracted great attention as high capacity information recording media. The optical recording media include rewritable media such as phase change type optical recording media and magneto-optical recording media and write-once media such as pit-constituting type optical recording media.
Among others, the phase change type optical recording media use recording film of a phase changeable alloy which changes its optical reflectance between crystalline and amorphous states or between two different crystalline states. For the phase change type optical recording media with different crystalline states, Ag--Zn alloy is a typical phase changeable alloy as described in Japanese Patent Application Kokai (JP-A) No. 130089/1986. This phase change type optical recording medium having Ag--Zn alloy as the recording film, however, shows a relatively small change in reflectivity due to the phase change. The compact disks (CD) and mini-disks (MD) which have been or will be used in practice rely on the reflectivity change mode in that information carrying areas have lower reflectivity than information-free areas. Since the Ag--Zn alloy when used as recording film provides another reflectivity change mode that light exposed areas, that is, recorded areas have increased reflectivity, it is difficult for media having a Ag--Zn alloy recording film to share a drive unit with the CDs and MDs.
For this and other reasons, JP-A 235789/1990 proposes a new optical recording disk satisfying the CD standard. Disclosed is an optical information recording disk having a high reflectivity layer and a low reflectivity layer stacked on an upper surface of a substrate in this order. The high reflectivity layer is formed of an element selected from the group consisting of Au, Al, Ag, Pt, Pd, Ni, Cr, and Co, or an alloy containing such an element or elements, which has high reflectivity of more than 70% with respect to incident laser light so that it cannot be a recording material as such. The low reflectivity layer is formed of a material which has high absorption to incident laser light over the wavelength range of 750 to 850 nm, for example, chalcogen elements such as Te. Information is recorded in this optical information recording disk by directing recording light to the disk from the substrate upper surface side, that is, the low reflectivity layer side, whereby the chalcogen element of the low reflectivity layer reacts with the element or elements of the high reflectivity layer to form a new alloy. In this way, the light exposed areas have reduced optical reflectivity. Such a change in optical reflectivity can be detected by directing reproducing light to the disk from the opposite side, that is, the substrate lower surface side. This configuration is described as providing write-once compact disks.
According to JP-A 235789/1990, the high and low reflectivity layers are formed by sputtering. We prepared optical recording disks of the disclosed configuration using a sputtering process and carried out recording and reproduction on the disks, finding that unrecorded areas had a reflectivity as low as about 14 to 16% and recorded areas had the reflectivity decreased to only about 10%. As a result, these disks could not be reproduced not only in the standard CD mode, but also by means of a driving equipment adapted for phase change type optical recording disks. This was because in forming a low reflectivity layer of Te on a high reflectivity layer of Ag by sputtering, interdiffusion occurred between the layers to form an alloy or compound between Ag and Te, suggesting that a recorded state was already established during sputtering. As a result, the magnitude of reflectivity and its change are small. It is to be noted that these results were obtained when the high reflectivity layer was formed to a thickness of about 500 .ANG. enough to carry out recording at a linear velocity of 1.2 to 1.4 m/s corresponding to the CD standard.
By increasing the thickness of the high reflectivity layer to about 1,000 .ANG., the influence of interdiffusion during formation of the low reflectivity layer was reduced so that sufficient reflection is provided by the high reflectivity layer in unrecorded state. However, in this case, it took a long time for the layers to diffuse into each other and recording could not be completed by irradiating recording laser light at the linear velocity prescribed by the CD standard.
In Example 5 of JP-A 235789/1990, a Sb layer and a Te layer are stacked on a high reflectivity layer of Au in this order as low reflectivity layers. A recorded state can be already established at the time of forming the Sb layer since interdiffusion readily takes place between Sb and Au.
A further problem arises with this optical information recording disk in which recording and reproducing light beams are directed to the disk from the substrate front and rear surface sides, respectively. This mode of recording requires that the disk be turned over and reversely rotated with the tracking polarity reversed. Thus a special drive is necessary for recording. Recording light is directed to the disk from the low reflectivity layer side because the high reflectivity layer has a high melting point and low absorption of incident light so that extremely high recording power is necessary if recording light is directed from the substrate side.